The present invention relates generally to bicycles and, more particularly, to a suspension system for a rear wheel of the bicycle with an adjustable suspension geometry that maintains a desired leverage ratio regardless of the geometric orientation of the suspension relative to a fixed shape of the bicycle frame.
The primary structural component of a conventional two-wheel bicycle is the frame. On a conventional road bicycle, the frame is typically constructed from a set of tubular members assembled together to form a fixed shape frame. For many bicycles, the frame is constructed from members commonly referred to as the top tube, down tube, seat tube, seat stays and chain stays, and those members are joined together at intersections commonly referred to as the head tube, seat post, bottom bracket and rear dropout. The top tube usually extends from the head tube rearward toward the seat tube. The head tube, sometimes referred to as the neck, is a short tubular structural member at the upper forward portion of the bicycle which supports the handlebar and front steering fork, which supports the front wheel. The down tube usually extends downwardly and rearward from the head tube toward the bottom bracket, the bottom bracket usually comprising a cylindrical member for supporting the pedals and flexible drive mechanism which from the drive train for powering the bicycle. The seat tube usually extends upward from the bottom bracket and is joined to a rear end of the top tube. The seat tube also usually functions to telescopically receive a seat post for supporting a seat or saddle for the bicycle rider to sit on.
The chain stays normally extend rearward from the bottom bracket. The seat stays normally extend downwardly and rearward from a location proximate a top of the seat tube. The chain stays and seat stays are normally joined together with a rear dropout for supporting the rear axle of the rear wheel. The portion of the frame defined by the head tube, seat post and bottom bracket and the structural members that join those three items together can be referred to as the main front triangular portion or a forward portion of the bicycle frame, with the seat stays and chain stays defining a rear triangular portion of the frame that is offset from the forward portion of the frame for supporting a rear wheel relative thereto. The foregoing description represents the construction of a conventional bicycle frame which generally does not possess a suspension system having any shock absorbing characteristics.
The increased popularity in recent years of off-road cycling, particularly on mountains and/or cross-country trails, has made a shock absorbing system in many instances a biking necessity. A bicycle with a properly designed suspension system is capable of traveling over extremely bumpy, uneven terrain and up or down very steep inclines. Suspension bicycles are less punishing, reduce fatigue and reduce the likelihood of injury to the rider, and are much more comfortable to ride over uneven terrain as compared to bicycles having what is commonly termed a fixed shape bicycle frame. For off-road cycling in particular, a suspension system greatly increases the rider's ability to control the bicycle because the wheels remain in contact with the ground as they ride over rocks and bumps in the terrain instead of being bounced into the air as occurs on conventional non-suspension bicycles.
Over the last several years the number of bicycles equipped with suspension systems has dramatically increased. In fact, many bicycles are now fully suspended, meaning that the bicycle has both a front wheel suspension system and a rear wheel suspension system. Front wheel suspensions were the first to become popular. Designed to mitigate the pounding associated with the bicycle front end, the front suspension is simpler to implement than a rear suspension. A front suspension fork is easy to retrofit onto older, fixed shape bicycle frame assemblies. On the other hand, rear suspension system increase traction and assist in cornering and balance the ride and do so particularly well over very uneven terrain.
During cycling, as the bicycle moves along a desired path, discontinuities of the terrain are communicated to the assembly of the bicycle and bicycle frame and ultimately to the rider. Although such discontinuities are generally negligible for cyclists operating on paved surfaces, riders venturing from the beaten path frequently encounter such terrain. With the proliferation of mountain biking, many riders seek the more treacherous trail. Technology has developed to assist such adventurous riders in conquering the road less traveled. Wheel suspension systems are one such feature.
Riding a fully suspended mountain bike along a rough, rock strewn trail, or even level riding on city and country roads, provides a new degree of comfort and capability to the rider. It is in downhill riding and racing that a rear suspension is most beneficial, but even on ordinary city and country roads, a rear suspension allows the rider to maintain a forward facing orientation to more safely view traffic and road conditions without paying disproportionate attention to stones and potholes immediately below in the rider's path.
A number of pivoting link suspensions have been developed for rear wheel suspensions on bicycles. In its simplest configuration, the chain stays, which on a conventional bicycle frame are rigidly mounted, are replaced by a pair of swing arms that are pivotably attached at their front ends to the generally fixed shape front triangular portion of the frame. The pivot is usually located near the bottom bracket where the pedal and crank are supported. The rear ends of the swing arms, which support the rear axle, move upward and downward in response to the rear wheel striking rocks, curbs and other obstructions. The range of movement of the swing arm usually is controlled by a shock, dampener, or shock absorber affixed between the swing arm and the main front frame portion and/or other members of the moveable rear wheel suspension linkage. Although such systems have allowed riders to conquer more aggressive terrain, room for improvement still exists.
Many riders appreciate that braking on mountain bikes can feel “chattery”, or as though the wheel is skipping over the terrain rather than rolling thereover. This chatter can detract from rider comfort and confidence as well as adversely affect bicycle performance. During normal operation, as the wheel moves across the ground, a contact patch of the tire is defined as the area of the tire that engages the ground surface. During translation of the suspension system relative to the frame, the contact patch rotates about the tire relative to an axis of rotation of the tire. Typically, the contact patch rotates 10 to 23 degrees for bikes having a suspension which travel ranging from about 122 to approximately 180 millimeters. Other suspension systems provide contact patch rotation in the range of 7 to 12 degrees for bicycles having 120 to 250 millimeters of suspension travel. Rotation of the contact patch contributes to the operational chatter perceived by the rider.
Braking forces as well as the relative orientation of the respective moveable links of the suspension system relative to each other and the underlying bicycle frame assembly also affect operation of the suspension system. With respect to the braking forces, the braking forces can cause the suspension system to compress or extend based, in part, on the orientation of the brake system with respect to the movable links of the suspension, and/or the orientation of the brake support relative to the fixed shape portion of the frame assembly constructed to support the rider, and/or the relative orientation of each of the moveable links relative to one another and/or relative the underlying fixed shaped frame portion of the bicycle assembly. Improper association of the brake system with the rear wheel and/or the movable members of the suspension system can detrimentally affect bicycle performance as well as stopping ability.
During braking, rider momentum generates a forward weight shift which acts to compress the front suspension while extending the rear suspension. The extension of the rear suspension system un-weights the rear wheel and tends to reduce rear tire traction. The reduction in rear tire traction adversely affects braking power in that, if the rear tire traction is sufficiently reduced, the rear tire may be allowed to slide along the ground surface. Such an event can distract a rider and may adversely affect the rider's ability to maintain control of the bicycle.
As alluded to above, suspension performance and rider comfort are also dependant on the orientation of the various movable links of the suspension system relative to each other and relative to the underlying fixed shape portion of the bicycle frame assembly. One parameter commonly considered during suspension system linkage design is the leverage ratio associated with the various discrete members of the linkage. Leverage ratio is a ratio that associates a movement experienced at a bicycle's rear axle to the movement experienced at a bicycle's suspension dampener, dampener, damper, or shock absorber. The leverage ratio is commonly graphically represented as a plot of instant leverage ratio vs. rear wheel travel wherein the instant leverage ratio is determined as the change in rear wheel travel divided by the change in dampener length. Suspension designers spend considerable time tuning this ratio to specific needs and performance requirements to satisfy various rider demands.
Commonly, adjustable geometry full suspension bicycles are designed to be somewhat adjustable so that riders can tune a suspension to provide a desired ride response. Unfortunately, those designs that provide adjustable geometry suspension systems result in undesired changes to the leverage ratio when the geometry of the suspension system is manipulated. That is, adjusting the relative position of the various links of the suspension relative to one another and/or the underlying bicycle frame assembly creates a deviation in the leverage ratio from a desired value associated with only the most common geometrical configuration of the suspension system. The manipulation of the geometry of such suspension systems detrimentally affects the leverage ratio such that the adjusted geometry provides less than desirable acceleration and deceleration response with respect to braking and contact patch response performance.
Accordingly, it is desired to provide an adjustable bicycle wheel suspension system that is geometrically adjustable but maintains a desired leverage ratio regardless of the relative orientation of the suspension system.